Method for operating a frequency converter circuit

ABSTRACT

A frequency converter circuit has at least two outputs that are respectively connected to a load, in particular an induction coil. A first output is operated at a first switching frequency and a second output is simultaneously operated at a second switching frequency that is different from the first, in such a way that noise having a frequency generated by the superposition of the first switching frequency and the second switching frequency is produced. The frequency converter circuit is operated in such a way that the frequency of the noise is lower than a first cutoff frequency and/or is higher than a second cutoff frequency.An induction cooking device includes first and second induction heating elements, and a frequency converter circuit including first and second power outputs electrically coupled to the first and second induction heating elements, respectively. The frequency converter circuit generates first and second power output signals with first and second switching frequencies applied to the first and second power outputs, respectively. The second switching frequency is selected to be no greater than a first cut-off frequency and no less than a second cut-off frequency (e.g., higher than the first cut-off frequency). The induction cooking device regulates electrical power applied to the first and/or second outputs by adjusting the switching frequency and/or a duration during which at least one of the first and second switching frequencies are applied in a repeating cycle during which the electrical power applied to the first and/or second outputs.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method for operating a frequencyconverter circuit comprising at least two outputs that are respectivelyconnected to a load, especially an induction coil, wherein a firstoutput is operated at a first switching frequency and a second output issimultaneously operated at a second switching frequency that isdifferent from the first in such a way that noise having a frequencygenerated by the superposition of the first switching frequency and thesecond switching frequency is produced.

Modern induction cooking surfaces are usually equipped with two or fourinduction cooking zones. The induction cooking zones have inductioncoils which are supplied with high-frequency operating currents by meansof converter circuits. It is known to operate two induction coilsjointly by means of one converter circuit with two outputs, each of theoutputs being connected to an induction coil. Various procedures havebeen proposed for avoiding or reducing noise when both outputs areoperated simultaneously.

Known from DE 196 54 268 C2 is a method for operating the convertercircuit where both outputs of the converter circuit are operated in timemultiplex so that no noise can occur. The disadvantage of this method isthat elaborate triggering and over-dimensioning of the power electronicsis required.

If the outputs are not operated in time multiplex and the two inductioncoils are supplied simultaneously with operating currents at differentfrequency, noise is produced. It is known to reduce this noise by meansof choking coils connected in series to the induction coil. Thedisadvantage of this method is that the method is not always stable. Inaddition, the noise can only be damped and the choking coils arerequired as additional components, making the converter circuit moreelaborate.

SUMMARY OF THE INVENTION

It is the object of the invention to provide an improved andcost-effective method for operating a converter circuit comprising atleast two outputs, especially for an induction cooking surface.

This object is solved by a method for operating a converter circuithaving the features of claim 1.

In a converter circuit comprising at least two outputs that arerespectively connected to a load, especially an induction coil, a firstoutput is operated at a first switching frequency and a second output issimultaneously operated at a second switching frequency that isdifferent from the first. In this way noise having a frequency generatedby the superposition of the first switching frequency and the secondswitching frequency is produced. The converter circuit is operated insuch a way that the frequency of the noise is lower than a first cut-offfrequency and/or higher than a second cut-off frequency. This procedurehas the advantage that noise can be produced at a frequency that liesoutside the human audible range by appropriately selecting the firstcut-off frequency and the second cut-off frequency. Furthermore, theinduction coils can be operated at frequencies at which a highefficiency can be achieved. In addition, additional components such aschoking coils for reducing the noise can be dispensed with.

According to a preferred embodiment, it is provided that the firstswitching frequency and/or the second switching frequency are operatedin such a way that the frequency of the noise is lower than the firstcut-off frequency and/or higher than the second cut-off frequency. Theswitching frequencies of the outputs can be simply adapted by means ofintelligent power switches.

Advantageously an electrical power of at least one of the outputs isregulated by means of a relative switch-on time and/or the switchingfrequency. Thus, the converter circuit can be operated with theinduction coils in such a way that a high efficiency is achieved.

According to a preferred embodiment, it is provided that the firstcut-off frequency and/or the second cut-off frequency are determineddepending on a level of the noise. In this way, the cut-off frequenciescan be adapted to the human audibility threshold so that the noisecannot be perceived.

In particular, the first cut-off frequency and/or the second cut-offfrequency are determined depending on a total electrical power of theoutputs. The level of the noise depends on the total electrical power ofthe outputs and the total electrical power can easily be determined. Inthis way, the cut-off frequencies can be adapted especially easily tothe human audibility threshold.

According to a preferred embodiment, it is provided that the firstcut-off frequency is 2 kilohertz and/or the second cut-off frequency is14 kilohertz. For these cut-off frequencies the human audibilitythreshold is very high so that the level of the noise does not reach thehuman audibility threshold or only insignificantly exceeds it.

In particular, the invention relates to an induction cooking device suchas, for example, an induction cooking surface or a cooker with aninduction heating element.

The invention and its further developments are explained in detailhereinafter with reference to drawings:

In the figures

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a first embodiment of a converter circuit,

FIG. 1b is a second embodiment of a converter circuit,

FIG. 2 is a schematic diagram of possible noise frequencies duringoperation of the converter circuits according to FIG. 1,

FIG. 3 is a schematic profile of the human audibility threshold,

FIG. 4 is a schematic time profile of a period of an output voltage ofthe converter circuits according to FIG. 1 and

FIG. 5 is a schematic diagram of an adaptation of electrical outputpowers for the converter circuits according to FIG. 1 taking intoaccount a first and a second cut-off frequency.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1a and 1b are schematic diagrams showing two different embodimentsof a converter circuit comprising two outputs or induction coils. Here Vdesignates a voltage source, I1 is a first and I2 is a second inductioncoil, S1, S2, S3 and S4 are high-frequency switches, CF1 and CF2 arecapacitive input filters and C1+, C1−, C2+ and C2− are capacitors. Thesecond embodiment (FIG. 1b) differs from the first embodiment (FIG. 1a)in that two changeover switches R1, R2 are provided for reconfiguringthe topology for the case when both induction coils I1, I2 are notswitched on or both outputs are not active.

FIG. 2 shows a schematic diagram of possible frequencies of the noiseduring operation of the converter circuits according to FIG. 1a or 1b.The first induction coil I1 is operated at a first switching frequencyf1 and the second induction coil I2 is operated at a second switchingfrequency f2 which is higher than the first switching frequency f1. Bothswitching frequencies f1, f2 lie above a maximum frequency f_(max) whichcan be perceived by human hearing. In this way, noise produced at theswitching frequencies f1 and f2 cannot be heard by humans. As a resultof a superposition of the two switching frequencies f1, f2, furthernoise is produced, for example, at a frequency fS which corresponds to adifference comprising the second switching frequency f2 minus the firstswitching frequency f1. This frequency fS can lie in a frequency band Bwhich indicates the frequencies perceptible by humans. The noise canhave different levels L1, L2, LS at different frequencies f1, f2, fSwhich is indicated by arrows of different length at the frequencies f1,f2 and fS in FIG. 2.

FIG. 3 shows a schematic profile of the human audibility threshold H.Depending on the frequency f, a different minimum noise level L can beperceived by the human hearing which is indicated by the profile of theaudibility threshold H in FIG. 3. A first cut-off frequency g1 and asecond cut-off frequency g2 are determined using the level LS of thenoise and its points of intersection with the profile of the audibilitythreshold H, the first cut-off frequency g1 being lower than the secondcut-off frequency g2. The converter circuits according to FIGS. 1a and1b are operated according to the invention so that the frequency fS ofthe noise is lower than the first cut-off frequency g1 or higher thanthe second cut-off frequency g2. In this way the noise is outside thehuman hearing range and thus cannot be perceived. The level LS of thepredicted noise can, for example, be estimated using the switchingfrequencies f1, f2 and the electrical powers P1 and P2 supplied to theinduction coils. Alternatively, experimental cut-off frequencies g1, g2can be defined, for example, the first cut-off frequency g1 at 2kilohertz and the second cut-off frequency g2 at 14 kilohertz.

Parameters for adapting the electrical powers P1, P2 supplied to theinduction coils I1, I2 are firstly the switching frequencies f1, f2 andsecondly a relative switch-on time D. FIG. 4 shows a schematic timeprofile of a period of a first output voltage UA of the convertercircuit according to FIGS. 1a and 1b. The period 1/f is normalised tounity in FIG. 4. The output voltage UA increases during the relativeswitch-on time D and then decreases slowly again. The electrical powersP1, P2 supplied to the induction coils I1, I2 are highest for relativeswitch-on times D of 0.5.

FIG. 5 shows a schematic diagram of an adaptation of the electricaloutput powers P1 and P2 for the two induction coils I1, I2 according tothe converter circuits from FIGS. 1a and 1b taking into account the twocut-off frequencies g1 and g2. For the first induction coil I1, forexample, which requires the higher electrical power P1 of the twoinduction coils I1, I2, the switching frequency f1 is specified as 21kilohertz for example and the relative switch-on time D is specified as0.5. The electrical power P2 for the second induction coil I2 is nowadjusted by means of the relative switch-on time D and by means of theswitching frequency f2 taking into account the two cut-off frequenciesg1 and g2. The second switching frequency f2 can lie in a range betweenthe first switching frequency f1 (here 21 kilohertz) and the sum of thefirst switching frequency f1 and the first cut-off frequency g1 (here 23kilohertz) and above the sum of the first switching frequency f1 and thesecond cut-off frequency g2 (here 35 kilohertz). In this way it isensured that the noise at the frequency fS which is produced from thedifference between the second switching frequency f2 and the firstswitching frequency f1 is not perceived by the human hearing.

REFERENCE LIST

-   B frequency band-   C1+ capacitor-   C1 capacitor-   C2+ capacitor-   C2 capacitor-   CF1 capacitive input filter-   CF2 capacitive input filter-   D relative switch-on time-   f frequency-   f_(max) maximum frequency perceived by human hearing-   f1 switching frequency of the first induction coil-   f2 switching frequency of the second induction coil-   fS frequency of the noise-   g1 first cut-off frequency-   g2 second cut-off frequency-   H audibility threshold-   I1 first induction coil-   I2 second induction coil-   L sound level-   L1 sound level at the first switching frequency-   L2 sound level at the second switching frequency-   LS level of noise at fS-   P electrical power-   P1 electrical power of the first induction coil-   P2 electrical power of the second induction coil-   R1 changeover switch-   R2 changeover switch-   t time-   U voltage-   UA output voltage-   V voltage source

1. A method of operating a frequency converter circuit having at leasttwo outputs respectively connected to a load, the method whichcomprises: operating a first output at a first switching frequency andsimultaneously operating a second output at a second switching frequencydifferent from the first switching frequency to produce noise having afrequency generated by a superposition of the first switching frequencyand the second switching frequency; operating the converter circuit toset the first switching frequency and the second switching frequencysuch that the frequency of the noise is lower than a first cut-offfrequency or higher than a second cut-off frequency, the second cut-offfrequency being higher than the first cut-off frequency; and regulatingan electrical power of at least one of the first and second outputs byadjusting the switching frequency and the relative switch-on time. 2.The method according to claim 1, wherein the load is an induction coil.3. The method according to claim 1, which comprises determining thefirst cut-off frequency and/or the second cut-off frequency independence on a level of the noise.
 4. The method of claim 3, furthercomprising the step of estimating a level of the noise using the firstand second switching frequencies of the first and second outputs and theelectrical power supplied to the loads.
 5. The method according to claim1, which comprises determining the first cut-off frequency and/or thesecond cut-off frequency in dependence on a total electrical power ofthe outputs.
 6. The method according to claim 1, which comprises settingthe first cut-off frequency at 2 kilohertz and/or setting the secondcut-off frequency at 14 kilohertz.
 7. The method of claim 1, wherein:the first switching frequency is determined according to the requiredelectrical power; the relative switch-on time of 0.5 is used for thefirst output; and the electrical power of the second load is adjustedusing the relative switch-on time and the second switching frequency,and taking into account the first and second cut-off frequencies.
 8. Themethod of claim 1, wherein the frequency of the noise corresponds to adifference between the second switching frequency and the firstswitching frequency.
 9. A cooktop including: a pair of induction coils;and a frequency converter circuit having a first output connected to afirst of the pair of induction coils and a second output connected to asecond of the pair of induction coils, the pair of induction coils beingeither individually or simultaneously operable, the first output beingassociated with a first switching frequency and the second output beingassociated with a second switching frequency, wherein, if the first andsecond switching frequencies are different from one another, noisehaving a noise frequency is generated when the pair induction coils isoperated simultaneously, wherein the first switching frequency and thesecond switching frequency are set or configured to have values suchthat the noise frequency is either above or below a human audibilitythreshold range associated with first and second cut-off frequencies,the second cut-off frequency being higher than the first cut-offfrequency, and wherein, when the first switching frequency is set for auser operated power setting and a selected switch-on time, the secondswitching frequency is selected to be in a range of frequenciesextending from the first switching frequency to a sum of the firstswitching frequency and the first cut-off frequency, and/or above a sumof the first switching frequency and the second cut-off frequency, thesecond switching frequency being associated with the selected switch-ontime.
 10. The cooktop of claim 9, wherein the switch-on time is 0.5. 11.A cooktop including: a pair of induction coils; and a frequencyconverter circuit having a first output connected to a first inductioncoil of the pair of induction coils and a second output connected to asecond induction coil of the pair of induction coils, the pair ofinduction coils being either individually or simultaneously operable,the first output being associated with a first switching frequency andthe second output being associated with a second switching frequency,wherein noise having a noise frequency is generated by superposition ofthe first switching frequency and the second switching frequency whenthe pair induction coils is operated simultaneously; wherein the firstswitching frequency and the second switching frequency are set to havevalues such that the noise frequency is either above or below a humanaudibility threshold range associated with first and second cut-offfrequencies, the second cut-off frequency being higher than the firstcut-off frequency, and wherein the frequency converter circuit isconfigured to control the first induction coil to operate at a firstswitching frequency for a user operated power setting at a selectedswitch-on time of 0.5, and the frequency converter circuit is configuredto determine the second switching frequency along the selected switch-ontime and taking into account the first and second cut-off frequencies.12. The cooktop of claim 11, wherein the second switching frequency is(1) equal to a sum of the first switching frequency and the firstcut-off frequency, and or (2) above a sum of the first switchingfrequency and the second cut-off frequency.
 13. A cooktop including: apair of induction heating units; and a frequency converter circuithaving a first output connected to a first induction heating unit of thepair of induction heating units and a second output connected to asecond induction heating unit of the pair of induction heating units,the pair of induction heating units being either individually orsimultaneously operable, the first output being associated with a firstswitching frequency and the second output being associated with a secondswitching frequency different from the first switching frequency inwhich case noise having a noise frequency is generated by superpositionof the first switching frequency and the second switching frequency whenthe pair induction heating units is operated simultaneously; wherein thefrequency converter circuit is configured to set or determine the firstswitching frequency and the second switching frequency such that thenoise frequency is generally outside a human audibility threshold range,or only insignificantly within it, during simultaneous operation,wherein the frequency converter circuit is configured to establish thefirst switching frequency for a first user selected power setting and aselected switch-on time, and to establish the second switching frequencyfor a second user selected power setting for the selected switch-ontime, and wherein the second switching frequency is (1) substantiallyequal to the first switching frequency, and/or (2) equal to or above asum of the first switching frequency and an upper cut-off frequencyassociated with the threshold range.
 14. The cooktop of claim 13,wherein the threshold range is associated with a lower cut-offfrequency.
 15. The cooktop of claim 14, wherein the upper cut-offfrequency is higher than the lower cut-off frequency.
 16. The cooktop ofclaim 13, wherein the switch-on time is 0.5.
 17. The cooktop of claim13, wherein each of the first and second heating elements comprises acoil.
 18. The cooktop of claim 13, not including a multiplexing unit.19. The cooktop of claim 13, not including a choking coil.
 20. Thecooktop of claim 13, further comprising a first power switch and asecond power switch associated, respectively, with the first and secondinductive heating units.
 21. The cooktop of claim 13, herein the firstand second switching frequencies lie above a maximum frequency that canbe perceived by a human.
 22. A converter circuit for an inductioncooking device having first and second induction coils, the convertercircuit comprising: an inlet connectable to a voltage source; a firstcircuit having a high frequency switch converting power from the voltagesource to a first high frequency output power applied to the firstinduction coil, wherein the first high frequency output power has afrequency (f1) corresponding to a switching frequency of the first highfrequency switch; a second circuit having a high frequency switchconverting power from the voltage source to a second high frequencyoutput power applied to the second induction coil, wherein the secondhigh frequency output power has a frequency (f2) corresponding to aswitching frequency of the second high frequency switch, wherein thedifference between the frequencies f2 and f1 is no greater than apredetermined lower threshold and is no less than a predetermined higherthreshold, and wherein the converter circuit regulates the power of thesecond high frequency output power by selecting the frequency f2 and aduration during which the second high frequency output power has thefrequency f2 in a repeating cycle applied to the second high frequencyoutput power.
 23. An induction cooking device comprising: a firstinduction heating element; a second induction heating element; afrequency converter circuit including a first power output electricallycoupled to the first induction heating element and a second power outputelectrically coupled to the second induction heating element; thefrequency converter circuit generating a first power output signalapplied to the first power output, wherein the first power output signalhas a first switching frequency; the frequency converter circuitgenerating a second power output signal applied to the second poweroutput, wherein the second power output signal has a second switchingfrequency; wherein the second switching frequency is selected by theinduction cooking device to be no greater than a first cut-off frequencyand no less than a second cut-off frequency, wherein the second cut-offfrequency is higher than the first cut-off frequency, and wherein theinduction cooking device regulates an electrical power applied to atleast one of the first and second outputs by adjusting the switchingfrequency and a duration during which at least one of the first andsecond switching frequencies are applied in a repeating cycle duringwhich the electrical power applied to the at least one of the first andsecond outputs.
 24. A cooktop including: a pair of induction coils; anda frequency converter circuit having a first output connected to a firstof the pair of induction coils and a second output connected to a secondof the pair of induction coils, the pair of induction coils being eitherindividually or simultaneously operable, the first output beingassociated with a first switching frequency and the second output beingassociated with a second switching frequency, wherein, if the first andsecond switching frequencies are different from one another, noisehaving a noise frequency is generated when the pair induction coils isoperated simultaneously, wherein the first switching frequency and thesecond switching frequency are set or configured to have values suchthat the noise frequency is either above or below a human audibilitythreshold range associated with first and second cut-off frequencies,the second cut-off frequency being higher than the first cut-offfrequency, and wherein, when the first switching frequency is set for auser operated power setting and the second switching frequency isselected to be in a range of frequencies extending from the firstswitching frequency to a sum of the first switching frequency and thefirst cut-off frequency, and/or above a sum of the first switchingfrequency and the second cut-off frequency, the second switchingfrequency being associated a selected duration during which the secondswitching frequency is applied to second output.
 25. A cooktopincluding: a pair of induction heating units; and a frequency convertercircuit having a first output connected to a first induction heatingunit of the pair of induction heating units and a second outputconnected to a second induction heating unit of the pair of inductionheating units, the pair of induction heating units being eitherindividually or simultaneously operable, the first output beingassociated with a first switching frequency and the second output beingassociated with a second switching frequency different from the firstswitching frequency in which case noise having a noise frequency isgenerated by superposition of the first switching frequency and thesecond switching frequency when the pair induction heating units isoperated simultaneously; wherein the frequency converter circuit isconfigured to set or determine the first switching frequency and thesecond switching frequency such that the noise frequency is generallyoutside a human audibility threshold range, or only insignificantlywithin it, during simultaneous operation, wherein the frequencyconverter circuit is configured to establish the first switchingfrequency for a first user selected power setting and to establish thesecond switching frequency for a second user selected power setting,wherein the second switching frequency being associated a selectedduration during which the second switching frequency is applied tosecond output, and wherein the second switching frequency is (1)substantially equal to the first switching frequency, and/or (2) equalto or above a sum of the first switching frequency and an upper cut-offfrequency associated with the threshold range.